from heapq import heappop,heappush

def main1(n,m,hwc):  
  # d0[i][j]:(0,0)から(i,j)への最小コスト
  # d1[i][j]:(i,j)から(n-1,n-1)への最小コスト

  cost=[0]*(n**2)
  for h,w,c in hwc:
    h,w=h-1,w-1
    cost[h*n+w]=c

  inf=float('inf')
  d0=[inf]*(n**2)
  d0[0]=0
  todo=[[0,0]]
  
  while todo:
    c,v=heappop(todo)
    if d0[v]<c:continue
    x,y=v//n,v%n
    for dx,dy in ((0,1),(1,0),(0,-1),(-1,0)):
      nx,ny=x+dx,y+dy
      if 0<=nx<n and 0<=ny<n:
        dc=cost[nx*n+ny]+1
        if d0[nx*n+ny]>c+dc:
          d0[nx*n+ny]=c+dc
          heappush(todo,[c+dc,nx*n+ny])
  
  d1=[inf]*(n**2)
  d1[(n-1)*n+n-1]=0
  todo=[[0,(n-1)*n+n-1]]
  while todo:
    c,v=heappop(todo)
    if d1[v]<c:continue
    x,y=v//n,v%n
    for dx,dy in ((0,1),(1,0),(0,-1),(-1,0)):
      nx,ny=x+dx,y+dy
      if 0<=nx<n and 0<=ny<n:
        dc=cost[nx*n+ny]+1
        if d1[nx*n+ny]>c+dc:
          d1[nx*n+ny]=c+dc
          heappush(todo,[c+dc,nx*n+ny])
  ans=inf
  for i in range(n**2):
    ans=min(ans,d1[i]+d0[i]-2*cost[i])
  return ans
if __name__=='__main__':
  n,m=map(int,input().split())
  hwc=[list(map(int,input().split())) for _ in range(m)]
  ret1=main1(n,m,hwc)
  print(ret1)